The traditional monolithic RF base transceiver station (BTS) architecture is increasingly being replaced by a distributed BTS architecture in which the functions of the BTS are separated into two physically separate units—a baseband unit (BBU) and a remote radio head (RRH). The BBU performs baseband processing for the particular air interface that is being used to wirelessly communicate over the RF channel. The RRH performs radio frequency processing to convert baseband data output from the BBU to radio frequency signals for radiating from one or more antennas coupled to the RRH and to produce baseband data for the BBU from radio frequency signals that are received at the RRH via one or more antennas.
The RRH is typically installed near the BTS antennas, often at the top of a tower, and the BBU is typically installed in a more accessible location, often at the bottom of the tower. The BBU and the RRH are typically connected through one or more fiber optic links. The interface between the BBU and the RRH is defined by front-haul communication link standards such as the Common Public Radio Interface (CPRI) family of specifications, the Open Base Station Architecture Initiative (OBSAI) family of specifications, and the Open Radio Interface (ORI) family of specifications.
Wireless operators are under constant pressure to increase the speed, capacity and quality of their networks while continuing to hold the line on cost. As technologies evolve, the challenge is becoming increasingly difficult. One specific reason: the escalating occurrence and cost of passive intermodulation (PIM) products in the uplink band.
Already recognized as a significant drain on network performance and profitability, the problem of PIM products is intensifying. PIM products are caused by nonlinearities in mechanical components of a wireless system, e.g., antenna connectors, junctions of dissimilar materials. PIM sources can also be found in nearby metal objects such as guy wires, anchors, roof flashings, and pipes. Also, rust, corrosion, loose connections, dirt, and oxidation may be a source of PIM products. Advanced wireless equipment is becoming more sensitive, and new technologies like LTE are increasingly noise limited. It has been noted that a 1 Decibel drop in uplink sensitivity due to PIM products in the uplink band can reduce coverage by as much as 11 percent.
Testing for PIM products using conventional coaxial RF testing equipment is slow, costly and dangerous. Each sector, frequency and technology must be individually connected and tested. So, most operators resort to PIM testing only after detecting a significant rise in the noise floor or a drop in connection quality.
One aspect of PIM testing involves determining the distance to a source of the PIM product. Conventionally, the distance to the PIM source is determined using a reflectometry system. This solution uses analog front end electronic components like a mixer, and an oscillator which provide good measurement accuracy. But this conventional system is dangerous in a distributed base station architecture with a digital interface like CPRI/OBSAI/ORI as a tower climb is used to connect to the antenna ports of the RRH.
Other solutions avoid the dangerous tower climb by estimating a delay for the RRH transceiver chain including the front-haul (optical fiber) communication link between the RRH and the BBU. This estimation is difficult to obtain with no knowledge of the CPRI/OBSAI/ORI protocol interface which can vary from manufacturer to manufacturer. Further, such estimates are prone to error due to the presence of time jitter, buffering in RRH delay, and low accuracy timing measurement at the CPRI/OBSAI/ORI interface. Therefore, there is a need in the art for a technique for safely and accurately determining the distance from an antenna port of an RRH to a PIM source.